Acanthomichelinoceras, Kröger, Klug & Mapes, 2005

Kröger, Björn, Klug, Christian & Mapes, Royal, 2005, Soft-tissue attachments in orthocerid and bactritid cephalopods from the Early and Middle Devonian of Germany and Morocco, Acta Palaeontologica Polonica 50 (2), pp. 329-342 : 338-339

Genus Acanthomichelinoceras nov.

Type species: Acanthomichelinoceras commutatum ( Giebel, 1852) . Derivation of the name: From acanthis (Lat.) = thorny, spiny, referring to its acute apex.

Diagnosis.—Medium sized, slightly compressed Michelinoceratinae with central siphuncle, long chambers and well−developed dorsal furrow (“Normallinie”). Very small, smooth, cap−shaped apex (dimension ca. 0.3 mm); cicatrix and primary constriction are absent.

Stratigraphic range.—Late Eifelian–early Givetian.

Remarks.—The mature growth stages of the new genus display the same morphology as those of Michelinoceras (Barrande 1866) . However, the new genus differs in the wider spacing of the septa, and the size and shape of the apex relative to other michelinoceratids. The apex of Acanthomichelinoceras measures only half of the diameter of Michelinoceras : it is blunt−ended, cup−shaped and significantly smaller in diameter (compare Ristedt 1968; Kiselev 1971; Serpagli and Gnoli 1977).

Attributed species.— Type species only.

Acanthomichelinoceras commutatum ( Giebel, 1852) Figs. 1C–E , 2A–D , 3B.

non Orthoceras gracile sp. nov.; Blumenbach 1803: pl. 2: 6. Orthoceras gracile Blumenbach ; Hoeninghaus 1830: 96.

Orthoceras gracile Blumenbach ; Beyrich 1837: 14.

Orthoceratites gracilis Blumenbach ; Quenstedt 1840: 268. Orthoceras regulare v. Schlotheim; d’Archiac and de Verneuil’s 1842: 344, pl. 27: 2.

Orthoceras regulare v. Schlotheim; Sandberger and Sandberger 1850 – 56: 173, pl. 20: 2.

* Orthoceras commutatum sp. nov.; Giebel 1852: 233.

Orthoceras regulare v. Schlotheim; Roemer 1862: 2.

Orthoceratites gracilis Blumenbach ; Quenstedt 1866: 407, pl. 34: 7. Orthoceras gracilis Blumenbach ; Roemer 1866: 225.

Orthoceras regulare v. Schlotheim; Maurer 1876: 830.

Orthoceras gracilis Blumenbach ; Maurer 1876: 830.

Orthoceras commutatum Giebel ; Kayser 1878: 69, pl. 10: 4, 8. Orthoceras longicameratum sp. nov. (non Orthoceras longicameratum Hall, 1859 ) [nom. nud.]; Foord 1888: 80.

Orthoceras nassovensis sp. nov. [nom. nud.]; Foord 1888: 80. Orthoceras commutatum Giebel ; Maurer 1896: 671–673.

Orthoceras aff. commutatum Giebel ; Paeckelmann 1913: 210–211. Orthoceras commutatum Giebel ; Dienst 1928: 108.

Syntypes: Sandberger and Sandberger (1850 –1856) in repository of NHW, original collection Sandberger, Kayser (1878) in repository of BGR/S.

Type horizon and locality: Wissenbacher Schiefer, early Eifelian, Middle Devonian.Wissenbach near Dietz in the Rhenish Massif, Germany.

Material.— A total of five specimens, two of them, specimens BGR /S t833 from the borehole Silberborn near Blankenburg , Harz mountains , and specimen GZG 1250 from Steinberg near Goslar , with preserved apex. The specimens MB.C.5366.1–5 come from the Scheibelsbach pit, Rupbachtal. The specimens NHW 56.9.05/402 and NHW 56.9.05/404 from Wissenbach in the Rhenish Massif preserve fragments of the phragmocone with the dorsal furrow .

Diagnosis.—As for genus.

Description.—The nearly mature growth stages of this species were described in detail by d’Archiac and de Verneuil (1842), Sandberger and Sandberger (1850 –1856), as well as Maurer (1896).

Apex.—Specimen GZG 1250 has a simple and blunt, cone− or cup−shaped apex ( Fig. 1C ); no ornamentation is preserved and a cicatrix is absent. The shell tip is not accentuated by different ornamentation or constriction from the rest of shell. The first chamber measures about 0.3 mm in diameter and is 0.25 mm long ( Table 2) whereas, the second chamber measures 0.25 mm. Specimen BGR /S t833 preserves the ten most apical chambers. The entire phragmocone measures 5.3 mm and the body chamber is 8.7 mm long; the phragmocone grows is 0.25 to 0.5 mm wide. Its tip is very poorly preserved, showing the principle dimensions of apex with a diameter of 0.25 mm. An apical constriction is missing.

# chamber 1 2 3 4 5 6 7 8 9 10 height 0.25 0.25 0.35 0.35 0.63 0.55 0.72 0.75 0.66 0.78

Remarks.—This species is one of the most common molluscs in the Wissenbacher Schiefer. The former name of the slate, “Orthoceras−Schiefer” ( Schubert 1996), refers to its abundance. Description of the Orthocerida began in the early 19 th century (see above). Because of the scarcity of characters in general and a lack of knowledge of the early growth stages, however, a genus designation was formerly impossible. A comparison with known michelinoceratids shows that the apex characters of these species are unique: The Llandoverian “ Orthoceratites ” described by Zhuravleva (1959: pl. 1: 6) resembles A. commutatum but it is about five times as large. The Wenlockian Michelinoceras michelini (Barrande, 1866) differs in its accentuated, cup−shaped apex as well as in its dimension by being about two times as large (e.g. Ristedt 1968: pl. 1: 1, 2; Histon 1999: pl. 2: 660). Notably the first chamber of Acanthomichelinoceras commutatum measures only 1/4 of the protoconch of Bactrites gracilis ( Blumenbach 1803; see Schindewolf 1933: pl. 3: 2–6).

Stratigraphic range and geographic occurrences.—Late Emsian–early Givetian, Harz mountains, Rhenish Massif, Bergisches Land ( Germany).

Figures

Fig. 1. Soft−tissue attachment structures in cephalopods from the Wissenbach Schiefer (early Eifelian). A. Bactrites gracile (Blumenbach, 1803), NHW 62.11/W−23/64, dorsal view, note the two different grayish bands (arrows) forming a lobe. B. Bactrites sp. A, NHW 402, dorsal view (B1), note two successive, slightly undulated grooves. Same specimen, lateral view (B). Same specimen, ventral view (B), note the small lobe in suture line. Same specimen, +

Fig. 2. Schematic outlines of the variability of the dorsal furrow in Acanthomichelinoceras commutatum (Giebel, 1852), showing the base of a chamber mould, with annular elevation dotted. Note the differences of the starting point of the dorsal furrow, sometimes within the mural area, sometimes within the annular elevation. A. NHW 56.9.05/402. B. NHW 56.9.05/404. C. MB.C.5366.4. D. MB.C.5366.1. Not to scale.